As shown in FIGS. 1 and 3, an existing Alstom gas turbine, the GT13E2, comprises an engine 10 receiving compressor gas into its plenum 11 in the direction 12. This gas is fed through a burner system 13 and into a combustion chamber 14 at lower pressure than the plenum 11, where it is combined with fuel and ignited. The lower pressure in the combustion chamber 14 means that the liner shell, comprising an inner liner shell 15 and an outer liner shell 17, both generally annular, have to withstand the differential pressures. In addition to the requirement to resist external pressure, the liner shells need to withstand high internal temperatures up to 500° C. or higher, and need to provide sufficient resistance to thermally-induced and pressure-induced stresses, creep and buckling failure modes which would otherwise result in an unacceptable component life. The shells need to be sufficiently rigid during operation and resistant to flexing during handling, to avoid damage to themselves and to any coatings applied to them. Cooling of the liner shells is usually provided in the form of impingement and/or convection cooling from the cold side of the shell wall. Channels or an annular cooling flow space are provided by an external structure, in the form of an exo-skeleton tile structure. A tile structure 16 of generally annular shape covers the inner liner shell 15, and correspondingly a similar tile structure 18 covers the outer liner shell 17.
As shown in FIG. 3a, which is a perspective view of parts of two adjacent tiles 18, linked edgewise parallel to the axial direction 25 of the engine, impingement flows 21 are caused by a multiplicity of apertures 32 through the tiles. Further, there are convection flows 31 along the annular gap between the cold side 19 of the liner shell and the exo-skeleton tile structure 18. The hot side of the liner shell 20 is heated by the combustion within the combustion chamber 14. The tiles 18 each have an edge strip 30 at a different radius from the remainder of the tile 18a, FIG. 3c, which accommodates the opposite edge of an adjacent tile 18b. The radial difference is the same as the thickness of the tile. This allows the adjacent tiles 18a, 18b to present a generally annular surface, even though they overlap. Retention tabs 28 are provided periodically along the edge to cover the edge strip 30, so as to retain the opposite edge of the adjacent tile 18b whilst allowing for circumferential expansion 29.
As shown in FIG. 3b, U clips 26, welded onto the hot side 20 of the liner shell 17, have integral studs which project through apertures 22 in the tiles 18. Nuts and Bellville washers 27 secure the studs in place, and locate the exo-skeleton tile structure over the liner shell 17.
This exo-skeleton tile structure resists bending in the axial and shear directions but has the disadvantage of having a low resistance to bending about the axially-extending edges of the adjacent tiles.
FIG. 2 is a series of graphs showing the temperature gradient and the thermal stresses resulting from a given constant thermal loading applied to liner shells of different wall thicknesses. The thermal stress is applied to a skin with a 1 mm TBC (Thermal Barrier Coating) on a high temperature turbine component which has active cooling. The coating is a ceramic type coating commonly containing Yttrium with a bond coat system. TBC provides the surface with additional temperature capability, acts as a reflector of radiation to reduce the overall heat flux and provides a small degree of insulation. There is convective cooling using a 1 mm rib height: a rib is provided on the cold side of the hot liner shell and acts as a turbulator to enhance the cooling convective heat transfer coefficient. Delta temperature, i.e. the difference in temperature across the skin, increases, as expected with wall thickness. Thermal stress also increases substantially with wall thickness. From this, it can be seen that there has to be a trade-off between component life, with respect to thermal stresses, on the one hand, and resistance to pressure buckling, on the other hand. A thin liner shell is preferred, for resisting thermal gradient stresses. However, resistance to buckling failure modes, particularly for the outer liner shell, is compromised by such thinner walls.
This explains the need for structural support external to the liner shell. The problem with the existing exo-skeleton tile structure with regard to this support is that, whilst it is capable of expansion in the circumferential direction, to accommodate changes in use, it offers little or no rigidity to bending in this circumferential direction.
Further, it is necessary to consider vibration modes in the gas turbine in use, and the existing configuration of exo-skeleton tile structure offers little opportunity for the tuning out of problematic resonances in the combined structure.
Accordingly, the purpose of the invention is to mitigate the disadvantages and limitations of the existing exo-skeleton tile structure.